Surface hardening is a process that is used to improve the wear resistance of articles and/or products, without affecting the softer, tougher interior of the articles. It will be appreciated that the combination of a hard surface and a resistance to cracking upon impact is extremely useful in articles, products and/or components such as cam or ring gears, bearings or shafts, turbine and/or automotive components, etc., as a very hard surface to resist wear in combination with a tough interior to resist the impacts that may occur during operation is often desirable for these kind of articles or components. Generally, a surface treatment of an article may result in compressive residual stresses at the article's surface that may reduce the probability of a crack initiation and arrest crack propagation at the case-core interface. Furthermore, surface hardening of steel may be advantageous over methods such as through hardening, because less expensive low-carbon and medium carbon steels can be surface hardened with minimal problems of distortion and cracking associated with through hardening of relatively thick sections.
Surface hardening may be achieved by means of diffusion methods, whereby the chemical composition of the surface may be modified with hardening elements such as carbon (C), nitrogen (N) or boron (B). Diffusion methods are beneficial in that they may provide an effective hardening of the entire surface of the articles to be processed.
Carburizing is the addition of carbon to the surface of a low-carbon steel at T=850-980° C., at which temperature austenite (face-centered cubic structure, FCC) is the stable crystal structure. Hardening is accomplished when the steel surface is quenched such that martensite (body-centered tetragonal structure, BCT) is formed.
In gas carburizing, the articles to be processed are surrounded by an atmosphere containing carbon. However, a problem related to this technique is that the composition of the atmosphere must be closely controlled to avoid deleterious side effects such as surface and grain-boundary oxides. In efforts to simplify the atmosphere, carburizing may instead be performed at very low pressures (vacuum carburizing). However, as the flow rate of the gas may be relatively low due to the low pressure, the carbon potential of the gas may be quickly depleted due to deep recesses and blind holes of the article material. This may result in a non-uniformity in case depth over the surface of the article. On the other hand, if the gas pressure is increased in order to overcome this problem, the problem of free-carbon formation (i.e. sooting) may arise. To obtain a reasonably uniform depth, the gas pressure must be increased periodically to replenish the depleted atmosphere, and then reduced again to avoid sooting, resulting in a highly complicated operation.
Hence, there is a wish for an alternative method which is able to provide a more convenient wear-resistance treatment of articles, products and/or objects, and which furthermore may be more cost-effective and/or time-effective.